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Minerals for the Ceramic Industry

The Principal mineral raw materials for the manufacture of ceramic products are the silica in different forms and the alumino-silicates.  These minerals, in appropriate combinations, are fused at high temperatures to obtain the required product.  While silica minerals include quartz and other forms like silica sand, the alumino-silicates comprise of the feldspars (orthoclase, microcline and albite) and clays.  In addition, limestone and also minor amounts of a large number of other minerals find use to certain extent in ceramics.

Silica:

For ceramic products, the quartz should be of good quality.  Iron staining and iron oxide minerals are considered as objectionable.  The important geological sources for silica suitable for the ceramic industry are the pegmatites (with well developed crystals of quartz), vein quartz, sandstones of high siliceous nature, high silica sands and orthoquartzites. 

In India, the major producing centres of silica are in the Sankargarh, Lohargarh and Bargarh regions (Uttar Pradesh), Bundi and Dausa in Jaipur and Adalpur in Sawai Madhopur (Rajasthan), Shimoga district (Karnataka), Burdwan district (West Bengal), Singbhum and Dhanbad districts (Bihar) and Guntur district (Andhra Pradesh).  Extensive deposits of quartzite suitable for glass manufacture occur in Mayurbhunj district of Orissa.  Some of the sandstones from Himmatnagar, Padharanali and Sankhera in Gujarat are suitable for the ceramic industry. 

Beneficiation:  The common impurities are clay, slime, iron stains and iron silicate minerals such as garnets.  Feldspar and mica are objection­able.  The raw material is ground in conventional manner and deslimed to remove the clayey fraction.  If iron is present as surface coating, this can be removed either by scrubbing or by chemical treatment (like heating in a dilute solution of titanous sulphate with some hydrofluoric acid).  Tabling can be adopted for the removal of clayey minerals and the heavy minerals like ilmenite and rutile.

The associated impurities such as mica and iron minerals can be removed by adopting flotation techniques.  The process is carried out in acid circuits (with pH between 2 and 3).  Mica is floated by using a combination of fuel of oil and some amine acetate.  Then the pulp is treated with one of the petroleum sulphonates and iron is removed in the subsequent stage of operation.  In the final stage, feldspar is floated by using hydrofluoric acid and amine acetate.

Feldspars:

Feldspars are used as fluxing material in the preparation of ceramic bodies, enamels and glazes.  Commonly potash feldspars (orthoclase and microcline) are used for this purpose and soda feldspar is chiefly used for glazing purposes.  Feldspars for use in ceramic industry should contain a minimum of 65-72% SiO2 and not less than 4% Na2O.

Feldspar is one of the dominant ingredients in acid igneous rocks, particularly of granitic and pegmatitic nature.  Pegmatites are often zoned into distinct bands of quartz and feldspar.  In India, the pegmatites of the famous mica belts in Rajasthan, Bihar and Andhra Pradesh are the major sources ceramic grade feldspars.  The other producing areas are in Salem and Tiruchirapalli districts of Tamil Nadu, Burdwan and Purulia districts of West Bengal and Hassan district of Karnataka.  Indian feldspars are exported to the U.K., France, W. Germany, Italy and Japan.

Beneficiation:  The common impurities to be eliminated are the iron-bearing minerals such as garnet and mica.  The quarried material is broken and feldspar is removed easily by handpicking.  The picked feldspar is reduced in size by crushing in conventional crushers and by subsequent grinding in pulverizers.  For glazing purposes and for the manufacture of ceramic ware, iron contamination during size reduc­tion is avoided by carrying out the grinding in pebble mills using flint pebbles as the grinding media.  The flotation techniques, similar to silica beneficiation, are also adopted for feldspar concentration.

Clays:

Clays have been classified on the basis of their physical properties and the industrial usage.  Of the various clays, ball and china clay are extensively used in ceramic industry.  China clay and ball clay belong to the kaolin group.  Although the primary constituent is kaolinite in both these clays, ball clay has greater plasticity and lesser refractoriness owing to the presence of mont­morillonite in considerable amounts.  It is normally added to china clay to achieve greater strength and the required plasticity.  These clays are a product of weathering processes of feldspathic rocks.  During weathering, the silica and iron oxides are partially leached with the residue (essentially of an aluminium silicate in composition) forming the in-situ deposits.  Depending upon the efficiency of the weathering process, impurities like grit (siliceous particles) and iron oxide minerals exist in different percent­ages in various clay deposits.

Ball Clay:  Ball Clays with high plasticity ranges have been reported from Khajwana, Indawar, jodhpur and Sheo areas in Rajasthan, Rampurda, Vagedia, Bagagela and Thoangadh areas in Gujarat, Kundra in Kerala, parts of Chingleput district in Tamil Nadu and Dwarka-Tirumala in Andhra Pradesh. 

China clay:  Usage of china clay for ceramic industry depends on factors such as plasticity, shrinkage (after drying and on firing), colour on firing and refractori­ness.  The grit content should normally be less than 1% and should never exceed 2%.

India has extensive deposits of china clays distributed in almost all the states.  However, good deposits are in Bhagalpur, Ranchi, Singbhum and Monghyr districts in Bihar; Mayurbhunj district in Orissa; Bankura and Birbhum districts in West Bengal; Banda district in Uttar Pradesh; Barmer, Pali, Bikaner and Ajmer districts in Rajasthan; Chingleput, North and South Arcot and Salem districts in Tamil Nadu; Adilabad, Anantapur, Nellore and Guntur districts of Andhra Pradesh; Chanda and Ratnagiri districts in Maharashtra; Shimoga and Hassan districts in Karnataka; Sabar­kantha district in Gujarat and Udhampur in Jammu and Kashmir. 

Beneficiation:  Major impurities in china clay are quartz, mica, felaspar and iron oxide minerals.  Methods such as sieving, washing, elutriation and levigation may be employed.  Normally the washing is done by 'levigation' process, which involves passing the clay slurry through a series of troughs or channels with different slopes.  This process aids in the settling of grit and other heavy mineral and floating of light fractions like mica.  Settling of finest quality clay takes place in the final tank.  Decolourising of the clay is also attempted for certain clays, which are coloured due to the presence of iron and titanium oxides.  However, no elaborate beneficiation techniques are employed in India and even the raw material that is marketed is not properly graded and specified.  This has been creating special problems to industries, particularly to those of smaller sizes.

 


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